The present invention relates to a spectrophotometer with a system for calibrating a monochromator. Particularly the system compensates for mechanical imperfections caused by constituent parts of the monochromator.
Monochromators generally used are described in, "GROUNDING AND METHOD OF SPECTROSCOPY", by Kelei Kudo pp. 405-413, Ohm Inc. (1985). In this related art, a diffraction grating is used as a wavelength dispersing element, and a pulse motor is used as means for driving diffraction grating. The rotation of the diffraction grating is controlled by the feed of a sine bar so as to make the rotation angle of the pulse motor proportional to the wavelength of a monochromatic beam extracted from an exit slit.
In a monochromator, the constituent parts such as a ball screw, a nut, a lever, a diffraction grating, and so on, are manufactured with sufficiently high accuracy, and if the positional relationship therebetween is adjusted properly, only one calibration wavelength is required within a measurable spectral range. If measurement is made as for the position on the ball screw of the nut where the light of the above calibration wavelength can be extracted from the exit slit, the quantity of rotation of the ball screw by which a desired wavelength can be obtained can be decided unequivocally with reference to the above measured position, so that wavelengths can be calibrated.
However, the manufacturing accuracy and the positional relationship of the constituent parts generally have errors to some extent. In monochromators which do not require high accuracy of wavelengths, it is possible to adopt a method in which an average value of errors of a wavelength measured on a plurality of calibration wavelengths is calculated to correct the measured wavelength by use of the calculated error, as disclosed in U.S. Pat. No. 4,779,216.
On the other hand, when higher accuracy wavelength setting is required, a plurality of calibration wavelengths are provided within a measurable spectral range, and errors between wavelength indication values of a monochromator in the respective calibration wavelengths and their real wavelengths are measured. There is used a wavelength calibration method in which a wavelength error function for estimating a wavelength error in a desired wavelength within the measurable spectral range is obtained on the basis of the measured errors, and wavelength indication values of the monochromator are corrected by this wavelength error function. For example, in the wavelength calibration method disclosed in Spectrochemics Acts, Vol. 39B, No. 7, pp. 867-878, 1984 or Applied Spectroscopy, Vol. 45, No. 6, pp. 993-998, 1991, wavelength errors with respect to a plurality of calibration wavelengths are measured to thereby calculate a wavelength error function constituted by a combination of quadratic and trigonometric functions. A measured wavelength is corrected by use of this calculated wavelength error function, so as to correct wavelength errors caused by the mismatching in length of a ball screw or a lever, the mismatching in relative positional relationship between a nut and the lever or the lever and a diffraction grating, or defects on the manufacture of the nut.
Another method to correct an error between a rotation angle indication value of a diffraction grating and its real angle is disclosed in JP-A-5-118922. In this method for measuring an error, light is incident to a monochromator from a light source having a plurality of known calibration wavelengths, and errors between obtained rotation angles of a diffraction grating and their theoretical rotation angles are calculated and stored on a table. When a real wavelength is measured, an obtained rotation angle of the diffraction grating is corrected by the errors stored on the table, and the wavelength of incident light is calculated from the corrected rotation angle.
Although it is not a method for correcting an error in wavelength measurement caused by the accuracy in manufacturing constituent parts or the like, "Apparatus for Automatically Reading Wavelength in a Monochromator" in which wavelength calibration and wavelength initial value setting at the time of turning on the power are performed rapidly by a simple configuration is disclosed in JP-A-62-207918. In this apparatus for automatically reading wavelength in a monochromator, a calibration wavelength .lambda..sub.c is made an initial setting value of a tuned wavelength of the monochromator at the time of turning on the power, and it is detected whether the tuned wavelength of the monochromator is larger or smaller than the calibration wavelength .lambda..sub.c. On the basis of the result of detection, the monochromator is scanned in the direction of the calibration wavelength .lambda..sub.c, and the value of a wavelength counter is set to the value .lambda..sub.c when the tuned wavelength becomes equal to the calibration wavelength .lambda..sub.c. Thus, wavelength calibration and wavelength initial value setting are carried out rapidly.
In the conventional methods for the wavelength calibration of a monochromator, it is assumed that any wavelength error occurs all over the measurable spectral range regularly, so that wavelengths are calibrated by a single wavelength error function the function form of which is known in advance.
It is indeed possible to attain effective correction with the above conventional wavelength calibration methods when a pitch error of a ball screw from its designed value is constant all over the available range of the screw or changes in accordance with an error function the function form of which is known in advance.
Generally, the pitch error of the ball screw is, however, not uniform all over the range. In addition, the pitch error of the ball screw does not always obey a specific function form all over the range even if it does not change suddenly within a narrow range. In such a case, effective calibration cannot be performed by the above conventional methods in which a single wavelength error function is applied to all over the measurable spectral range. The same applies to the defect in flatness of the end surfaces of a lever and a nut which slidably rub against each other in a sine bar mechanism.